Resistive memory device having a retention layer with non-linear ion conductivity

ABSTRACT

A memory device is disclosed. The memory device includes a bottom contact and a memory layer connected to the bottom contact, where the memory layer has a variable resistance. The device also includes a conductive top electrode on the memory layer, where the top electrode and the memory layer cooperatively form a heterojunction memory structure. The device also includes a retention layer between the memory layer and the top electrode, where the retention layer has an ionic conductivity which varies non-linearly with voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/924,014, entitled “A RESISTIVE MEMORY DEVICEHAVING A RETENTION LAYER,” filed Mar. 16, 2018, which is incorporatedherein by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to nonvolatile memory devices,and more particularly to memory devices having effective speedcomparable to DRAM, which do not require speed-crippling errorcorrection and include hetero junctions of oxide materials.

BACKGROUND OF THE INVENTION

In general, memory devices or systems can be segmented in 3 distinctcategories: internet-of-things (IoT) memories, embedded memories, andhigh-density high-volume memories. The memory requirements (cost,density, speed, endurance, retention, power consumption) are quitedifferent for each of these 3 categories.

IoT memories tend to be inexpensive, power-efficient, and low-density.Memories embedded in complex system chips tend to be fast,area-efficient, and medium-density. High-density high-volume memoriesmust be scalable to small geometries to be cost effective.

The high-density high-volume memory category is currently dominated byDRAM (which is volatile) and NAND Flash (which is non-volatile).

DRAM is very-fast, exhibits exceptional endurance, and is therefore bestsuited for fast system memory. DRAM, however, is expensive and volatile(for example, the data may need to be refreshed every 60 milliseconds)and sacrifices retention to maximize speed and endurance.

In sharp contrast, NAND Flash is inexpensive with much higher bitcapacity and good retention, and is best suited for low-cost siliconstorage. NAND Flash, however, sacrifices both speed and endurance tomaximize retention.

Being limited to two dimensions (2D), DRAM will likely remain expensivesince silicon area largely defines cost per gigabyte. In contrast, thecost of NAND Flash is expected to decline over time because of threedimensional (3D) stacking. The cost gap between DRAM and NAND Flash willlikely increase over time.

DRAM and NAND Flash fit their sweet spots near perfectly and it seemshighly unlikely that a universal memory combining the best of DRAM andNAND Flash will ever exist. It is equally unlikely that any emergingmemory technology will replace DRAM because its speed and endurancecombination is exceptionally hard to beat. Furthermore, there is noeconomic justification to build a NAND Flash replacement forhigh-density applications while NAND Flash prices continue to decrease.

However, as data processing and storage needs continue their rapidincrease for mobile devices and cloud data centers, the industry needs anew non-volatile memory with attributes much closer to DRAM (because itis impossible to replace) than to NAND Flash (because it does not needto be replaced).

This vast space between DRAM and NAND Flash is therefore an opportunityfor innovation.

Storage Class Memory is an emerging non-volatile memory segmentpositioned between the most successful system memory (DRAM) and the mostsuccessful silicon storage (NAND Flash). There are many opportunitiesfor new memories in the vast space between DRAM and NAND Flash, eachwith different speed, endurance and retention metrics.

The biggest opportunities are always where the difficulty is greatestand that is in the space closest to DRAM. The ultimate market demand istherefore for Storage Class Memory with DRAM speed, the highestendurance achievable with this speed, a cost per gigabyte closer to NANDFlash, and a pragmatic retention far superior to DRAM retention.

Furthermore, certain semiconductor memory technologies have applied aprincipal of geometric redundancy, where multiple data bits may bestored in a single cell. This property of a memory cell to support amultiple of values is sometimes referred to as its dynamic range. Todate the for memory cells have abilities to support a dynamic rangeanywhere between 1 and 4 bits. These combined properties ofsemiconductors have increased capacities and reduced costs.

Another issue associated with semiconductor memory manufacturing hasbeen the substantial costs of the semiconductor foundries which can bemore than a billion dollars to establish. Amortizing expenses increasethe cost of memory chips. Now, with advances in foundry resolutionsenabling smaller cell sizes and the geometric redundancy of multiplebit-level per memory cell semiconductor memory is actually cheaper perunit cost, and substantially more rugged in terms of high G forces thanmemory files on a disk drive.

In Flash memories, there have been improvements, but they have becomesusceptible to write cycle limitations and ability to support dynamicranges are diminished as the quantum limit is approached. Another issuewith Flash memory is its limitations in write speeds and the number ofwrite cycle limitations the cell will tolerate before it permanentlyfails.

Accordingly, what is desired is a memory system and method whichovercomes the above-identified problems. The systems and methods shouldbe easily implemented, cost effective, and adaptable to existing storageapplications.

BRIEF SUMMARY OF THE INVENTION

One general aspect includes a memory device, including a bottomelectrode. The memory device also includes a memory layer connected tothe bottom electrode, where the memory layer has a variable resistance.The memory device also includes a conductive top electrode on the memorylayer, where the top electrode and the memory layer cooperatively form aheterojunction memory structure. The memory device also includes aretention layer between the memory layer and the top electrode, wherethe retention layer has a variable ionic conductivity, where theretention layer is configured to selectively resist ionic conduction,and where the resistivity of the retention layer is less than 1×10-4ohm-m.

Implementations may include one or more of the following features. Thememory device further including a template layer between the bottomelectrode and the memory layer, where the crystalline structure of thememory layer matches the crystalline structure of the template layer.The memory device where the conductivity of the template layer isgreater than 10×106 s m-1. The memory device where a first contactformed at an interface between the retention layer and the memory layeris ohmic, and where a second contact formed at an interface between theretention layer and the top electrode is ohmic. The memory devicefurther including: a first barrier layer, configured to substantiallyprevent the conduction of ions or vacancies therethrough, where the topelectrode is between the first barrier layer and the retention layer;and a second barrier layer, configured to substantially prevent theconduction of ions or vacancies therethrough, where the memory layer isbetween the second barrier layer and the retention layer. The memorydevice where the first and second barrier layers each have a resistivityless than 1 e-4 ohm-m. The memory device further including a sidebarrier layer, where the first and second barrier layers and the sidebarrier layer define an enclosed space, where the top electrode, theretention layer, and the memory layer are within the space, and whereions of the top electrode, the retention layer, and the memory layer areconfined to the space by the first and second barrier layers and theside barrier layer.

Another general aspect includes a method of manufacturing a memorydevice, including: forming a bottom electrode. The method also includesconnecting a memory layer to the bottom electrode, where the memorylayer has a variable resistance. The method also includes forming aconductive top electrode on the memory layer, where the top electrodeand the memory layer cooperatively form a heterojunction memorystructure. The method also includes forming a retention layer betweenthe memory layer and the top electrode, where the retention layer has avariable ionic conductivity, where the retention layer is configured toselectively resist ionic conduction, and where the resistivity of theretention layer is less than 1×10-4 ohm-m.

Implementations may include one or more of the following features. Themethod further including forming a template layer between the bottomelectrode and the memory layer, where the crystalline structure of thememory layer matches the crystalline structure of the template layer.The method where the conductivity of the template layer is greater than10×106 s m-1. The method where a first contact formed at an interfacebetween the retention layer and the memory layer is ohmic, and where asecond contact formed at an interface between the retention layer andthe top electrode is ohmic. The method further including: forming afirst barrier layer, configured to substantially prevent the conductionof ions or vacancies therethrough, where the top electrode is betweenthe first barrier layer and the retention layer; and forming a secondbarrier layer, configured to substantially prevent the conduction ofions or vacancies therethrough, where the memory layer is between thesecond barrier layer and the retention layer. The method where the firstand second barrier layers each have a resistivity less than 1 e-4 ohm-m.The method further including forming a side barrier layer, where thefirst and second barrier layers and the side barrier layer define anenclosed space, where the top electrode, the retention layer, and thememory layer are within the space, and where ions of the top electrode,the retention layer, and the memory layer are confined to the space bythe first and second barrier layers and the side barrier layer.

Another general aspect includes a method of using a memory device, thememory device including a bottom electrode, a memory layer with avariable resistance connected to the bottom electrode, a conductive topelectrode on the memory layer, and a retention layer between the memorylayer and the top electrode, where the top electrode and the memorylayer cooperatively form a heterojunction memory structure, where theretention layer has a variable ionic conductivity, where the retentionlayer is configured to selectively resist ionic conduction, and wherethe resistivity of the retention layer is less than 1×10-4 ohm-m. Themethod includes applying a first voltage difference across the bottomelectrode and the top electrode, where an electric field is generated inthe memory layer, and such that a resistivity state of the memory layeris changed. The method also includes applying a second voltagedifference across the bottom electrode and the top electrode. The methodalso includes while the second voltage difference is applied, causing afirst current to be conducted through the bottom electrode, the memorylayer, the retention layer, and the top electrode. The method alsoincludes determining the resistivity state of the memory layer based onthe second voltage and the first current.

Implementations may include one or more of the following features. Themethod where the memory device further includes a template layer betweenthe bottom electrode and the memory layer, where the crystallinestructure of the memory layer matches the crystalline structure of thetemplate layer, and where the first current is additionally conductedthrough the template layer. The method where the conductivity of thetemplate layer is greater than 10×106 s m-1. The method where the memorydevice further includes: a first barrier layer, configured tosubstantially prevent the conduction of ions or vacancies therethrough,where the top electrode is between the first barrier layer and theretention layer; and a second barrier layer, configured to substantiallyprevent the conduction of ions or vacancies therethrough, where thememory layer is between the second barrier layer and the retentionlayer, where the method further includes causing the first current to beconducted through the first and second barrier layers. The method wherethe first and second barrier layers each have a resistivity less than 1e-4 ohm-m. The method where the memory device further includes a sidebarrier layer, where the first and second barrier layers and the sidebarrier layer define an enclosed space, where the top electrode, theretention layer, and the memory layer are within the space, and wherethe method further includes confining ions of the top electrode and thememory layer to the space with the first and second barrier layers andthe side barrier layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a memory device according to anembodiment.

FIG. 2 is a schematic illustration of a memory device according to anembodiment.

FIG. 3 is a schematic illustration of a memory device according to anembodiment.

FIG. 4 is a schematic illustration of a memory device according to anembodiment.

FIG. 5 is a schematic illustration of a memory device according to anembodiment.

FIG. 6 is a schematic illustration of a memory device according to anembodiment.

FIG. 7 is a schematic illustration of a memory device according to anembodiment.

FIG. 8 is a schematic illustration of a memory device according to anembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Particular embodiments of the invention are illustrated herein inconjunction with the drawings.

Various details are set forth herein as they relate to certainembodiments.

However, the invention can also be implemented in ways which aredifferent from those described herein. Modifications can be made to thediscussed embodiments by those skilled in the art without departing fromthe invention. Therefore, the invention is not limited to particularembodiments disclosed herein.

The present invention is related to a nonvolatile memory device. Thememory device can be utilized in a variety of applications from a freestanding nonvolatile memory to an embedded device in a variety ofapplications. These applications include but are not limited to embeddedmemory used in a wide range of SOC (system on chip), switches inprogrammable or configurable ASIC, solid state drive used in computersand servers, memory sticks used in mobile electronics like camera, cellphone, iPod® etc. The memory device comprises a first metal layer and afirst metal oxide layer coupled to the first metal layer. The memorydevice includes a second metal oxide layer coupled to the first metaloxide layer and a second metal layer coupled to the second metal oxidelayer. These metal and metal oxide layers can be of a variety of typesand their use will be within the spirit and scope of the presentinvention. More particularly, some of the embodiments disclosed hereinwill include PCMO as one of the metal oxide layers. It is wellunderstood by one of ordinary skill in the art that the presentinvention should not be limited to this metal oxide layer or any otherlayer disclosed herein, as other metal oxide layers may alternatively beused.

FIG. 1 is an illustration of a memory device 100 which includes aconductive Platinum (Pt) bottom contact 180, which is coupled to aPraseodymium Calcium Manganese Oxide (PCMO) memory layer 150, which iscoupled to a metal top electrode layer 130.

Top electrode layer 130 forms an electrical connection between thememory layer 150 and another device. Top electrode layer 130 is formedwith a material which forms a secure bond with the memory layer 150.

Top electrode layer 130 cooperatively forms a metal oxide heterojunctionmemory with memory layer 150, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 150 in response to anelectric field applied across the electrode layer 130 and the memorylayer 150. In some embodiments, the top electrode layer 130 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 150. In alternative embodiments, the topelectrode layer 130 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 150.

As understood by those of skill in the art, the resistivity of thememory layer 150 is dependent on the concentration of oxygen ions orvacancies therein. Therefore, memory device 100 functions as arewritable memory cell, where the state of the memory device correspondswith the resistivity of the memory layer 150. The memory layer 150 iswritten by applying a voltage to induce an electric field to force theconcentration of the oxygen ions or vacancies to a desired concentrationstate, and the desired concentration state corresponds with a desiredresistivity state. As a result, the resistance of the memory layer isprogrammed by the write operation. To read the state of the memory cell,a voltage or a current may be applied to the cell. A current or voltagegenerated in response to the applied voltage or current is dependent onthe resistance state of the memory cell, and may be sensed to determinethe resistance state.

FIG. 2 is a schematic illustration of a memory device 200 according toan embodiment. Memory device 200 includes bottom contact 280, conductivebottom barrier layer 270, template layer 260, memory layer 250, optionalretention layer 240, top electrode layer 230, top barrier layer 220, topcontact 210, and side barrier layer 290. In some embodiments, sidebarrier layer 290 is substantially annular and surrounds bottom contact280, conductive bottom barrier layer 270, template layer 260, memorylayer 250, retention layer 240 (if present), top electrode layer 230,top barrier layer 220, and top contact 210.

Memory device 200 may be formed by forming bottom contact 280, formingconductive bottom barrier layer 270 on bottom contact 280, formingtemplate layer 260 on conductive bottom barrier layer 270, formingmemory layer 250 on template layer 260, optionally forming retentionlayer 240 on memory layer 250, forming top electrode layer 230 onretention layer 240 or on memory layer 250, forming top barrier layer220 on top electrode layer 230, forming top contact 210 on top barrierlayer 220, and forming side barrier layer 290 on both lateral sides ofeach of bottom contact 280, conductive bottom barrier layer 270,template layer 260, memory layer 250, retention layer 240 (if present),top electrode layer 230, top barrier layer 220, and top contact 210.

In some embodiments, each of the interfaces of the various layers ofmemory device 200 forms an ohmic contact between the layers.

In some embodiments, top contact 210 includes at least one of Copper(Cu), Aluminum (Al), Tungsten (W), Ruthenium (Ru), Platinum (Pt),Iridium (Ir), and Rhodium (Rh). In alternative embodiments, one or moreother materials are used.

Top contact 210 is used to form an electrical connection between thememory device 200 and other electrical components. Top contact 200 mayalso be used to form a mechanical connection between the memory device200 and another device.

In some embodiments, top barrier layer 220 includes at least one ofTitanium Nitride (TiN), Tantalum Nitride (TaN), Titanium AluminumNitride (TiAlN), Tantalum Aluminum Nitride (TaAlN), Titanium SiliconNitride (TiSiN), Tantalum Silicon Nitride (TaSiN), and Titanium Tungsten(TiW). In alternative embodiments, one or more other materials are used.

Top barrier layer 220 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 230, anyretention layer 240, and the memory layer 250. Top barrier layer 220 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 200. Accordingly, topbarrier layer 220 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 230 into the top barrier layer220. In addition, top barrier layer 220 is configured to conductelectrical current between the top electrode layer 230 and the topcontact 210. For example, top barrier layer 220 may have a resistivityless than 1 E-4 ohm-m.

The top barrier layer 220 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 220experiences substantially no chemical reaction with the top electrode230, such that the characteristics of the top barrier layer 220 and thetop electrode 230 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 220 and the top electrode 230, such that thecharacteristics of the memory layer 250 and the retention layer 240remain substantially unaffected by one another.

In some embodiments, top electrode layer 230 includes at least one ofTungsten (W), Molybdenum (Mo), Nickel (Ni), Iron (Fe), Cobalt (Co), andChromium (Cr). In alternative embodiments, one or more other materialsare used. For example, another metal, conductive oxide, or otherconductive compound may be use.

Top electrode layer 230 forms an electrical connection between theretention layer 240 or the memory layer 250 and the top barrier layer220. Top electrode layer 230 is formed with a material which forms asecure bond with the retention layer 240 or the memory layer 250.

Top electrode layer 230 cooperatively forms a metal oxide heterojunctionmemory with memory layer 250, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 250 in response to anelectric field applied across the electrode layer 230 and the memorylayer 250. In some embodiments, the top electrode layer 230 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 250. In alternative embodiments, the topelectrode layer 230 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 250.

In some embodiments, optional retention layer 240 includes at least oneof SnOx, InOx, (IN,SN)Ox, and doped ZnO. In alternative embodiments, oneor more other materials are used.

In some embodiments, retention layer 240 has high electricalconductivity. For example, retention layer 240 may have a resistivityless than 1 E-4 ohm-m. Retention layer 240 may also be selectivelyresistant to conduction of oxygen ions and vacancies in response to anapplied electric field. In addition, voltage dependence of the ionicconductivity of retention layer 240 may be highly non-linear.Furthermore, retention layer 240 may experience no chemical interactionwith the top electrode layer 230 and memory layer 250. Additionally,retention layer 240 may form an ohmic contact with top electrode 230.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 230and the memory layer 250. Retention layer 240 may be placed between thetop electrode layer 230 and the memory layer 250 and improves memorycell retention. Because retention layer 240 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 240 side of topelectrode layer 230 and the memory layer 250, and data retention isimproved. In addition, because retention layer 240 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 240.

The retention layer 240 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 240experiences substantially no chemical reaction with the memory layer250, such that the characteristics of the memory layer 250 and theretention layer 240 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 240 and the memory layer 250, such that thecharacteristics of the memory layer 250 and the retention layer 240remain substantially unaffected by one another.

In some embodiments, memory layer 250 includes at least one ofPraseodymium Calcium Manganese Oxide or (Pr1-xCax)MnO₃ (PCMO),(Sm1-xCax)MnO₃, and (La1-xSrx)MnO₃. In alternative embodiments, one ormore other materials are used. In some embodiments, the memory layer 250is between about 5 nm and about 10 nm thick.

In some embodiments, template layer 260 includes at least one of LaNiO₃,NdNiO₃, SrRuO₃, CaRuO₃, and LaMnO₃. In alternative embodiments, one ormore other materials are used.

The electrical conductivity of the template layer 260 is similar toconductivity of commonly used metallic bottom electrodes, such as Ru.For example, the electrical conductivity of the template layer 260 maybe greater than about 10×10⁶ S m⁻¹. In some embodiments, the electricalconductivity of the template layer 260 is greater than about 15×10⁶ Sm⁻¹, is greater than about 20×10⁶ S m⁻¹, is greater than about 30×10⁶ Sm⁻¹, or is greater than about 50×10⁶ S m⁻¹. In addition, the crystallinestructure and lattice parameters of the template layer 260 are similarto those of the memory layer 250. For example, the crystalline structureand lattice parameters of the template layer match the crystallinestructure and lattice parameters of the memory layer 250. Consequently,misfit stresses between the template layer 260 and the memory layer 250are less than that which would occur in the memory layer 250 if thememory layer 250 were formed directly on the bottom barrier 270.

In some embodiments, the template layer 260 behaves as a latency layerat least partly because of its low resistivity. Accordingly, theresistance of the memory device 200 is lowered. This, combined with theeffect of the retention layer 240 and the high on/off resistance ratio,increases the memory window, such that low read voltages may be used.For example, the read voltage can be about 0.5V, about 0.4V, 0.3V, 0.2V,0.1V or lower.

The template layer 260 may be formed using any deposition process, suchas physical vapor deposition (PVD), chemical vapor deposition (CVD),sputtering, evaporation, atomic layer deposition (ALD), or anotherdeposition or growth process.

In some embodiments, memory layer 250 may be epitaxially grown ontemplate layer 260. In some embodiments, the memory layer 250 is formedinto thin films (e.g. epitaxially grown crystalline thin films) on thetemplate layer 260 at temperatures lower than 450 C. In someembodiments, the temperature while forming the template layer 260 may be400 C or less, 350 C or less, 300 C or less, 250 C or less, or 200 C orless. Because of the low temperature while forming the template layer260, the template layer 260 may be formed as part of a CMOSmanufacturing process.

Furthermore, in some embodiments, template layer 260 experiencessubstantially no chemical reaction with the memory layer 250, such thatthe characteristics of the memory layer 250 remain substantiallyunaffected by the template layer 260. Also, in some embodiments,substantially no diffusion occurs between the template layer 260 and thememory layer 250, such that the characteristics of the memory layer 250remain substantially unaffected by the template layer 260.

In some embodiments, the crystalline film of the memory layer 250 may begrown on an amorphous template layer 260 acting as a growth seed. Insome embodiments, the crystalline film of the memory layer 250 may begrown on a crystalline template layer 260 acting as a seed. When thememory layer 250 is grown, the ambient environment (e.g., Ar and O₂) mayhave a pressure between 9 and 10 torr. In some embodiments, water isremoved from the ambient environment.

In some embodiments, when the memory layer 250 is formed on the templatelayer 260, no or substantially no amorphous memory layer 250 orinterface layer is formed at the interface between the memory layer 250and the template layer 260. Accordingly, the thickness of the memorylayer 250 is reduced, which is beneficial for high density devices.

The typical on/off resistance ratio (the ratio of the resistance of theon or low resistance state of the memory device 200 to the resistance ofthe off or high resistance state of the memory device 200) for interfaceswitching material films is not amenable for multi-bit storage in asingle cell. However, in embodiments such as that illustrated in FIG. 2,because of the substantially defect free interface between the memorylayer 250 and the template layer 260 and because of the high qualitycrystalline structure of the memory layer 250, few, if any, oxygen ionsare trapped by crystal defects, such that substantially all of theoxygen ions are free to migrate between the memory layer 250 and the topelectrode 230, and the on/off resistance ratio of the memory device 200is maximized. For example, the on/off resistance ratio may be 2 orgreater, 5 or greater, 10 or greater, 20 or greater, 35 or greater, 50or greater, 75 or greater, or 100 or greater.

In some embodiments, conductive bottom barrier layer 270 includes atleast one of Titanium Nitride (TiN), Tantalum Nitride (TaN), TitaniumAluminum Nitride (TiAlN), Tantalum Aluminum Nitride (TaAlN), TitaniumSilicon Nitride (TiSiN), Tantalum Silicon Nitride (TaSiN), and TitaniumTungsten (TiW). In alternative embodiments, one or more other materialsare used. In some embodiments, conductive bottom barrier layer 270 isformed of substantially the same material as the top barrier layer 220.

Bottom barrier layer 270 may be formed of a material having a band gapwider than that of one or more of the template layer 260, any retentionlayer 240, and the memory layer 250. Bottom barrier layer 270 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 200. Accordingly, bottombarrier layer 270 substantially prevents oxygen ions or vacancies fromescaping from the template layer 260 into the bottom barrier layer 270.In addition, bottom barrier layer 270 is configured to conductelectrical current between the template layer 260 and the bottom contact280. For example, bottom barrier layer 270 may have a resistivity lessthan 1 E-4 ohm-m.

The bottom barrier layer 270 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, bottom barrier layer270 experiences substantially no chemical reaction with the bottomcontact 280, such that the characteristics of the bottom barrier layer270 and the bottom contact 280 remain substantially unaffected by oneanother. Also, in some embodiments, substantially no diffusion occursbetween the bottom barrier layer 270 and the bottom contact 280, suchthat the characteristics of the bottom barrier layer 270 and the bottomcontact 280 remain substantially unaffected by one another.

In some embodiments, bottom contact 280 includes at least one of Copper(Cu), Aluminum (Al), Tungsten (W), Ruthenium (Ru), Platinum (Pt),Iridium (Ir), and Rhodium (Rh). In alternative embodiments, one or moreother materials are used. In some embodiments, bottom contact 280 isformed of substantially the same material as the top contact 210.

In some embodiments, side barrier 290 includes at least one of AlOx,SiO₂, and Si₃N₄. In alternative embodiments, one or more other materialsare used.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 200, top barrier layer 220, bottom barrier layer 270,and side barrier layers 290 have little or substantially zero oxygen iondiffusion coefficients, such that the oxygen ions and vacancies areconfined to top electrode layer 230, retention layer 240 (if present),memory layer 250, and template layer 260 by top barrier layer 220,bottom barrier layer 270, and side barrier layers 290. As a result, thereliability of memory device 200 is excellent.

The side barrier layers 290 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, side barrier layers290 experience substantially no chemical reaction with the other layers,such that the characteristics of the side barrier layers 290 and theother layers remain substantially unaffected by one another. Also, insome embodiments, substantially no diffusion occurs between the sidebarrier layers 290 and the other layers, such that the characteristicsof the side barrier layers 290 and the other layers remain substantiallyunaffected by one another.

In certain embodiments, bottom contact 280 is formed with Cu, conductivebottom barrier layer 270 is formed with TaN, template layer 260 isformed with LaNiO₃, memory layer 250 is formed with PCMO, retentionlayer 240 is formed with SnO, top electrode layer 230 is formed with W,top barrier layer 220 is formed with TaN, and top contact 210 is formedwith Cu.

In certain embodiments, bottom contact 280 is formed with Ru, conductivebottom barrier layer 270 is formed with TaN, template layer 260 isformed with SrRuO₃, memory layer 250 is formed with PCMO, retentionlayer 240 is formed with doped ZnO, top electrode layer 230 is formedwith W, top barrier layer 220 is formed with TaN, and top contact 210 isformed with Ru.

In certain embodiments, bottom contact 280 is formed with W, conductivebottom barrier layer 270 is formed with TaN, template layer 260 isformed with CaRuO₃, memory layer 250 is formed with (SmCa)MnO₃,retention layer 240 is formed with InOx, top electrode layer 230 isformed with W, top barrier layer 220 is formed with TaN, and top contact210 is formed with Cu.

FIG. 3 is a schematic illustration of a memory device 300 according toan embodiment. Memory device 300 includes bottom contact 380, conductivebottom barrier layer 370, template layer 360, memory layer 350, optionalretention layer 340, top electrode layer 330, top barrier layer 320, andtop contact 310.

Memory device 300 may be formed by forming bottom contact 380, formingconductive bottom barrier layer 370 on bottom contact 380, formingtemplate layer 360 on conductive bottom barrier layer 370, formingmemory layer 350 on template layer 360, optionally forming retentionlayer 340 on memory layer 350, forming top electrode layer 330 onretention layer 340 or on memory layer 350, forming top barrier layer320 on top electrode layer 330, and forming top contact 310 on topbarrier layer 320.

In some embodiments, each of the interfaces of the various layers ofmemory device 300 forms an ohmic contact between the layers.

Top contact 310 may have characteristics similar or identical to thoseof top contact 210 discussed elsewhere herein.

Top contact 310 is used to form an electrical connection between thememory device 300 and other electrical components. Top contact 300 mayalso be used to form a mechanical connection between the memory device300 and another device.

Top barrier layer 320 may have characteristics similar or identical tothose of top barrier layer 220 discussed elsewhere herein.

Top barrier layer 320 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 330, anyretention layer 340, and the memory layer 350. Top barrier layer 320 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 300. Accordingly, topbarrier layer 320 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 330 into the top barrier layer320. In addition, top barrier layer 320 is configured to conductelectrical current between the top electrode layer 330 and the topcontact 310.

The top barrier layer 320 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 320experiences substantially no chemical reaction with the top electrode330, such that the characteristics of the top barrier layer 320 and thetop electrode 330 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 320 and the top electrode 330, such that thecharacteristics of the memory layer 350 and the retention layer 340remain substantially unaffected by one another.

Top electrode layer 330 may have characteristics similar or identical tothose of top electrode layer 230 discussed elsewhere herein.

Top electrode layer 330 forms an electrical connection between theretention layer 340 or the memory layer 350 and the top barrier layer320. Top electrode layer 330 is formed with a material which forms asecure bond with the retention layer 340 or the memory layer 350.

Top electrode layer 330 cooperatively forms a metal oxide heterojunctionmemory with memory layer 350, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 350 in response to anelectric field applied across the electrode layer 330 and the memorylayer 350. In some embodiments, the top electrode layer 330 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 350. In alternative embodiments, the topelectrode layer 330 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 350.

Optional retention layer 340 may have characteristics similar oridentical to those of optional retention layer 240 discussed elsewhereherein.

In some embodiments, retention layer 340 may experience no chemicalinteraction with the top electrode layer 330 and memory layer 350.Additionally, retention layer 340 may form an ohmic contact with topelectrode 330.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 330and the memory layer 350. Retention layer 340 may be placed between thetop electrode layer 330 and the memory layer 350 and improves memorycell retention. Because retention layer 340 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 340 side of topelectrode layer 330 and the memory layer 350, and data retention isimproved. In addition, because retention layer 340 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 340.

The retention layer 340 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 340experiences substantially no chemical reaction with the memory layer350, such that the characteristics of the memory layer 350 and theretention layer 340 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 340 and the memory layer 350, such that thecharacteristics of the memory layer 350 and the retention layer 340remain substantially unaffected by one another.

Memory layer 350 may have characteristics similar or identical to thoseof memory layer 250 discussed elsewhere herein.

Template layer 360 may have characteristics similar or identical tothose of template layer 260 discussed elsewhere herein.

The electrical conductivity of the template layer 360 is similar toconductivity of commonly used metallic bottom electrodes, such as Ru. Inaddition, the crystalline structure and lattice parameters of thetemplate layer 360 are similar to those of the memory layer 350.Consequently, misfit stresses between the template layer 360 and thememory layer 350 are minimized.

In some embodiments, the template layer 360 behaves as a latency layerat least partly because of its low resistivity. Accordingly, theresistance of the memory device 300 is lowered. This, combined with theeffect of the retention layer 340 and the high on/off resistance ratio,increases the memory window, such that low read voltages may be used.For example, the read voltage can be about 0.5V, about 0.4V, 0.3V, 0.2V,0.1V or lower.

The template layer 360 may be formed using any deposition process, suchas physical vapor deposition (PVD), chemical vapor deposition (CVD),sputtering, evaporation, atomic layer deposition (ALD), or anotherdeposition or growth process.

In some embodiments, memory layer 350 may be epitaxially grown ontemplate layer 360. In some embodiments, the memory layer 350 is formedinto thin films (e.g. epitaxially grown crystalline thin films) on thetemplate layer 360 at temperatures lower than 450 C. In someembodiments, the temperature while forming the template layer 360 may be400 C or less, 350 C or less, 300 C or less, 350 C or less, or 300 C orless. Because of the low temperature while forming the template layer360, the template layer 360 may be formed as part of a CMOSmanufacturing process.

Furthermore, in some embodiments, template layer 360 experiencessubstantially no chemical reaction with the memory layer 350, such thatthe characteristics of the memory layer 350 remain substantiallyunaffected by the template layer 360. Also, in some embodiments,substantially no diffusion occurs between the template layer 360 and thememory layer 350, such that the characteristics of the memory layer 350remain substantially unaffected by the template layer 360.

In some embodiments, the crystalline film of the memory layer 350 may begrown on an amorphous template layer 360 acting as a growth seed. Insome embodiments, the crystalline film of the memory layer 350 may begrown on a crystalline template layer 360 acting as a seed. When thememory layer 350 is grown, the ambient environment (e.g., Ar and O₂) mayhave a pressure between 9 and 10 torr. In some embodiments, water isremoved from the ambient environment.

In some embodiments, when the memory layer 350 is formed on the templatelayer 360, no or substantially no amorphous memory layer 350 orinterface layer is formed at the interface between the memory layer 350and the template layer 360. Accordingly, the thickness of the memorylayer 350 is reduced, which is beneficial for high density devices.

The typical on/off resistance ratio (the ratio of the resistance of theon or low resistance state of the memory device 300 to the resistance ofthe off or high resistance state of the memory device 300) for interfaceswitching material films is not amenable for multi-bit storage in asingle cell. However, in embodiments such as that illustrated in FIG. 3,because of the substantially defect free interface between the memorylayer 350 and the template layer 360 and because of the high qualitycrystalline structure of the memory layer 350, few, if any, oxygen ionsare trapped by crystal defects, such that substantially all of theoxygen ions are free to migrate between the memory layer 350 and the topelectrode 330, and the on/off resistance ratio of the memory device 300is maximized. For example, the on/off resistance ratio may be 2 orgreater, 5 or greater, 10 or greater, 20 or greater, 35 or greater, 50or greater, 75 or greater, or 100 or greater.

Conductive bottom barrier layer 370 may have characteristics similar oridentical to those of conductive bottom barrier layer 270 discussedelsewhere herein. In some embodiments, conductive bottom barrier layer370 is formed of substantially the same material as the top barrierlayer 320.

Bottom barrier layer 370 may be formed of a material having a band gapwider than that of one or more of the template layer 360, any retentionlayer 340, and the memory layer 350. Bottom barrier layer 370 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 300. Accordingly, bottombarrier layer 370 substantially prevents oxygen ions or vacancies fromescaping from the template layer 360 into the bottom barrier layer 370.In addition, bottom barrier layer 370 is configured to conductelectrical current between the template layer 360 and the bottom contact380.

The bottom barrier layer 370 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, bottom barrier layer370 experiences substantially no chemical reaction with the bottomcontact 380, such that the characteristics of the bottom barrier layer370 and the bottom contact 380 remain substantially unaffected by oneanother. Also, in some embodiments, substantially no diffusion occursbetween the bottom barrier layer 370 and the bottom contact 380, suchthat the characteristics of the bottom barrier layer 370 and the bottomcontact 380 remain substantially unaffected by one another.

Bottom contact 380 may have characteristics similar or identical tothose of conductive bottom contact 280 discussed elsewhere herein. Insome embodiments, bottom contact 380 is formed of substantially the samematerial as the top contact 310.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 300, top barrier layer 320 and bottom barrier layer 370have little or substantially zero oxygen ion diffusion coefficients,such that the oxygen ions and vacancies are confined to top electrodelayer 330, retention layer 340 (if present), memory layer 350, andtemplate layer 360 by top barrier layer 320 and bottom barrier layer370. As a result, the reliability of memory device 300 is excellent.

In certain embodiments, bottom contact 380 is formed with Cu, conductivebottom barrier layer 370 is formed with TaN, template layer 360 isformed with LaNiO₃, memory layer 350 is formed with PCMO, retentionlayer 340 is formed with SnO, top electrode layer 330 is formed with W,top barrier layer 320 is formed with TaN, and top contact 310 is formedwith Cu.

In certain embodiments, bottom contact 380 is formed with Ru, conductivebottom barrier layer 370 is formed with TaN, template layer 360 isformed with SrRuO₃, memory layer 350 is formed with PCMO, retentionlayer 340 is formed with doped ZnO, top electrode layer 330 is formedwith W, top barrier layer 320 is formed with TaN, and top contact 310 isformed with Ru.

In certain embodiments, bottom contact 380 is formed with W, conductivebottom barrier layer 370 is formed with TaN, template layer 360 isformed with CaRuO₃, memory layer 350 is formed with (SmCa)MnO₃,retention layer 340 is formed with InOx, top electrode layer 330 isformed with W, top barrier layer 320 is formed with TaN, and top contact310 is formed with Cu.

FIG. 4 is a schematic illustration of a memory device 400 according toan embodiment. Memory device 400 includes template layer 460, memorylayer 450, optional retention layer 440, top electrode layer 430, topbarrier layer 420, and top contact 410.

Memory device 400 may be formed by forming template layer 460, formingmemory layer 450 on template layer 460, optionally forming retentionlayer 440 on memory layer 450, forming top electrode layer 430 onretention layer 440 or on memory layer 450, forming top barrier layer420 on top electrode layer 430, and forming top contact 410 on topbarrier layer 420.

In some embodiments, each of the interfaces of the various layers ofmemory device 400 forms an ohmic contact between the layers.

Top contact 410 may have characteristics similar or identical to thoseof top contact 210 discussed elsewhere herein.

Top contact 410 is used to form an electrical connection between thememory device 400 and other electrical components. Top contact 410 mayalso be used to form a mechanical connection between the memory device400 and another device.

Top barrier layer 420 may have characteristics similar or identical tothose of top barrier layer 220 discussed elsewhere herein.

Top barrier layer 420 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 430, anyretention layer 440, and the memory layer 450. Top barrier layer 420 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 400. Accordingly, topbarrier layer 420 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 430 into the top barrier layer420. In addition, top barrier layer 420 is configured to conductelectrical current between the top electrode layer 430 and the topcontact 410.

The top barrier layer 420 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 420experiences substantially no chemical reaction with the top electrode430, such that the characteristics of the top barrier layer 420 and thetop electrode 430 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 420 and the top electrode 430, such that thecharacteristics of the memory layer 450 and the retention layer 440remain substantially unaffected by one another.

Top electrode layer 430 may have characteristics similar or identical tothose of top electrode layer 230 discussed elsewhere herein.

Top electrode layer 430 forms an electrical connection between theretention layer 440 or the memory layer 450 and the top barrier layer420. Top electrode layer 430 is formed with a material which forms asecure bond with the retention layer 440 or the memory layer 450.

Top electrode layer 430 cooperatively forms a metal oxide heterojunctionmemory with memory layer 450, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 450 in response to anelectric field applied across the electrode layer 430 and the memorylayer 450. In some embodiments, the top electrode layer 430 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 450. In alternative embodiments, the topelectrode layer 430 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 450.

Optional retention layer 440 may have characteristics similar oridentical to those of optional retention layer 240 discussed elsewhereherein.

In some embodiments, retention layer 440 may experience no chemicalinteraction with the top electrode layer 430 and memory layer 450.Additionally, retention layer 440 may form an ohmic contact with topelectrode 430.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 430and the memory layer 450. Retention layer 440 may be placed between thetop electrode layer 430 and the memory layer 450 and improves memorycell retention. Because retention layer 440 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 440 side of topelectrode layer 430 and the memory layer 450, and data retention isimproved. In addition, because retention layer 440 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 440.

The retention layer 440 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 440experiences substantially no chemical reaction with the memory layer450, such that the characteristics of the memory layer 450 and theretention layer 440 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 440 and the memory layer 450, such that thecharacteristics of the memory layer 450 and the retention layer 440remain substantially unaffected by one another.

Memory layer 450 may have characteristics similar or identical to thoseof memory layer 250 discussed elsewhere herein.

Template layer 460 may have characteristics similar or identical tothose of template layer 260 discussed elsewhere herein.

The electrical conductivity of the template layer 460 is similar toconductivity of commonly used metallic bottom electrodes, such as Ru. Inaddition, the crystalline structure and lattice parameters of thetemplate layer 460 are similar to those of the memory layer 450.Consequently, misfit stresses between the template layer 460 and thememory layer 450 are minimized.

In some embodiments, the template layer 460 behaves as a latency layerat least partly because of its low resistivity. Accordingly, theresistance of the memory device 400 is lowered. This, combined with theeffect of the retention layer 440 and the high on/off resistance ratio,increases the memory window, such that low read voltages may be used.For example, the read voltage can be about 0.5V, about 0.4V, 0.3V, 0.2V,0.1V or lower.

The template layer 460 may be formed using any deposition process, suchas physical vapor deposition (PVD), chemical vapor deposition (CVD),sputtering, evaporation, atomic layer deposition (ALD), or anotherdeposition or growth process.

In some embodiments, memory layer 450 may be epitaxially grown ontemplate layer 460. In some embodiments, the memory layer 450 is formedinto thin films (e.g. epitaxially grown crystalline thin films) on thetemplate layer 460 at temperatures lower than 450 C. In someembodiments, the temperature while forming the template layer 460 may be400 C or less, 450 C or less, 400 C or less, 450 C or less, or 400 C orless. Because of the low temperature while forming the template layer460, the template layer 460 may be formed as part of a CMOSmanufacturing process.

Furthermore, in some embodiments, template layer 460 experiencessubstantially no chemical reaction with the memory layer 450, such thatthe characteristics of the memory layer 450 remain substantiallyunaffected by the template layer 460. Also, in some embodiments,substantially no diffusion occurs between the template layer 460 and thememory layer 450, such that the characteristics of the memory layer 450remain substantially unaffected by the template layer 460.

In some embodiments, the crystalline film of the memory layer 450 may begrown on an amorphous template layer 460 acting as a growth seed. Insome embodiments, the crystalline film of the memory layer 450 may begrown on a crystalline template layer 460 acting as a seed. When thememory layer 450 is grown, the ambient environment (e.g., Ar and O₂) mayhave a pressure between 9 and 10 torr. In some embodiments, water isremoved from the ambient environment.

In some embodiments, when the memory layer 450 is formed on the templatelayer 460, no or substantially no amorphous memory layer 450 orinterface layer is formed at the interface between the memory layer 450and the template layer 460. Accordingly, the thickness of the memorylayer 450 is reduced, which is beneficial for high density devices.

The typical on/off resistance ratio (the ratio of the resistance of theon or low resistance state of the memory device 400 to the resistance ofthe off or high resistance state of the memory device 400) for interfaceswitching material films is not amenable for multi-bit storage in asingle cell. However, in embodiments such as that illustrated in FIG. 4,because of the substantially defect free interface between the memorylayer 450 and the template layer 460 and because of the high qualitycrystalline structure of the memory layer 450, few, if any, oxygen ionsare trapped by crystal defects, such that substantially all of theoxygen ions are free to migrate between the memory layer 450 and the topelectrode 430, and the on/off resistance ratio of the memory device 400is maximized. For example, the on/off resistance ratio may be 2 orgreater, 5 or greater, 10 or greater, 20 or greater, 35 or greater, 50or greater, 75 or greater, or 100 or greater.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 400, top barrier layer 420 has little or asubstantially zero oxygen ion diffusion coefficient, such that theoxygen ions and vacancies are confined to top electrode layer 430,retention layer 440 (if present), memory layer 450, and template layer460 by top barrier layer 420. As a result, the reliability of memorydevice 400 is excellent.

FIG. 5 is a schematic illustration of a memory device 500 according toan embodiment. Memory device 500 includes template layer 560, memorylayer 550, optional retention layer 540, and top electrode layer 530.

Memory device 500 may be formed by forming template layer 560, formingmemory layer 550 on template layer 560, optionally forming retentionlayer 540 on memory layer 550, and forming top electrode layer 530 onretention layer 540.

In some embodiments, each of the interfaces of the various layers ofmemory device 500 forms an ohmic contact between the layers.

Top electrode layer 530 may have characteristics similar or identical tothose of top electrode layer 230 discussed elsewhere herein.

Top electrode layer 530 forms an electrical connection between theretention layer 540 and other electrical components. Top electrode layer530 may also be used to form a mechanical connection between the memorydevice 500 and another device. Top electrode layer 530 is formed with amaterial which forms a secure bond with the retention layer 540.

Top electrode layer 530 cooperatively forms a metal oxide heterojunctionmemory with memory layer 550, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 550 in response to anelectric field applied across the electrode layer 530 and the memorylayer 550. In some embodiments, the top electrode layer 530 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 550. In alternative embodiments, the topelectrode layer 530 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 550.

Optional retention layer 540 may have characteristics similar oridentical to those of optional retention layer 240 discussed elsewhereherein.

In some embodiments, retention layer 540 may experience no chemicalinteraction with the top electrode layer 530 and memory layer 550.Additionally, retention layer 540 may form an ohmic contact with topelectrode 530.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 530and the memory layer 550. Retention layer 540 may be placed between thetop electrode layer 530 and the memory layer 550 and improves memorycell retention. Because retention layer 540 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 540 side of topelectrode layer 530 and the memory layer 550, and data retention isimproved. In addition, because retention layer 540 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 540.

The retention layer 540 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 540experiences substantially no chemical reaction with the memory layer550, such that the characteristics of the memory layer 550 and theretention layer 540 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 540 and the memory layer 550, such that thecharacteristics of the memory layer 550 and the retention layer 540remain substantially unaffected by one another.

Memory layer 550 may have characteristics similar or identical to thoseof memory layer 250 discussed elsewhere herein.

Template layer 560 may have characteristics similar or identical tothose of template layer 260 discussed elsewhere herein.

The electrical conductivity of the template layer 560 is similar toconductivity of commonly used metallic bottom electrodes, such as Ru. Inaddition, the crystalline structure and lattice parameters of thetemplate layer 560 are similar to those of the memory layer 550.Consequently, misfit stresses between the template layer 560 and thememory layer 550 are minimized.

In some embodiments, the template layer 560 behaves as a latency layerat least partly because of its low resistivity. Accordingly, theresistance of the memory device 500 is lowered. This, combined with theeffect of the retention layer 540 and the high on/off resistance ratio,increases the memory window, such that low read voltages may be used.For example, the read voltage can be about 0.5V, about 0.4V, 0.3V, 0.2V,0.1V or lower.

The template layer 560 may be formed using any deposition process, suchas physical vapor deposition (PVD), chemical vapor deposition (CVD),sputtering, evaporation, atomic layer deposition (ALD), or anotherdeposition or growth process.

In some embodiments, memory layer 550 may be epitaxially grown ontemplate layer 560. In some embodiments, the memory layer 550 is formedinto thin films (e.g. epitaxially grown crystalline thin films) on thetemplate layer 560 at temperatures lower than 550 C. In someembodiments, the temperature while forming the template layer 560 may be500 C or less, 550 C or less, 500 C or less, 550 C or less, or 500 C orless. Because of the low temperature while forming the template layer560, the template layer 560 may be formed as part of a CMOSmanufacturing process.

Furthermore, in some embodiments, template layer 560 experiencessubstantially no chemical reaction with the memory layer 550, such thatthe characteristics of the memory layer 550 remain substantiallyunaffected by the template layer 560. Also, in some embodiments,substantially no diffusion occurs between the template layer 560 and thememory layer 550, such that the characteristics of the memory layer 550remain substantially unaffected by the template layer 560.

In some embodiments, the crystalline film of the memory layer 550 may begrown on an amorphous template layer 560 acting as a growth seed. Insome embodiments, the crystalline film of the memory layer 550 may begrown on a crystalline template layer 560 acting as a seed. When thememory layer 550 is grown, the ambient environment (e.g., Ar and O₂) mayhave a pressure between 9 and 10 torr. In some embodiments, water isremoved from the ambient environment.

In some embodiments, when the memory layer 550 is formed on the templatelayer 560, no or substantially no amorphous memory layer 550 orinterface layer is formed at the interface between the memory layer 550and the template layer 560. Accordingly, the thickness of the memorylayer 550 is reduced, which is beneficial for high density devices.

The typical on/off resistance ratio (the ratio of the resistance of theon or low resistance state of the memory device 500 to the resistance ofthe off or high resistance state of the memory device 500) for interfaceswitching material films is not amenable for multi-bit storage in asingle cell. However, in embodiments such as that illustrated in FIG. 5,because of the substantially defect free interface between the memorylayer 550 and the template layer 560 and because of the high qualitycrystalline structure of the memory layer 550, few, if any, oxygen ionsare trapped by crystal defects, such that substantially all of theoxygen ions are free to migrate between the memory layer 550 and the topelectrode 530, and the on/off resistance ratio of the memory device 500is maximized. For example, the on/off resistance ratio may be 2 orgreater, 5 or greater, 10 or greater, 20 or greater, 35 or greater, 50or greater, 75 or greater, or 100 or greater.

FIG. 6 is a schematic illustration of a memory device 600 according toan embodiment. Memory device 600 includes bottom contact 680, conductivebottom barrier layer 670, memory layer 650, optional retention layer640, top electrode layer 630, top barrier layer 620, top contact 610,and side barrier layer 690. In some embodiments, side barrier layer 690is substantially annular and surrounds bottom contact 680, conductivebottom barrier layer 670, memory layer 650, retention layer 640 (ifpresent), top electrode layer 630, top barrier layer 620, and topcontact 610.

Memory device 600 may be formed by forming bottom contact 680, formingconductive bottom barrier layer 670 on bottom contact 680, formingmemory layer 650 on conductive bottom barrier layer 670, optionallyforming retention layer 640 on memory layer 650, forming top electrodelayer 630 on retention layer 640 or on memory layer 650, forming topbarrier layer 620 on top electrode layer 630, forming top contact 610 ontop barrier layer 620, and forming side barrier layer 690 on bothlateral sides of each of bottom contact 680, conductive bottom barrierlayer 670, memory layer 650, retention layer 640 (if present), topelectrode layer 630, top barrier layer 620, and top contact 610.

In some embodiments, each of the interfaces of the various layers ofmemory device 600 forms an ohmic contact between the layers.

In some embodiments, top contact 610 includes at least one of Copper(Cu), Aluminum (Al), Tungsten (W), Ruthenium (Ru), Platinum (Pt),Iridium (Ir), and Rhodium (Rh). In alternative embodiments, one or moreother materials are used.

Top contact 610 is used to form an electrical connection between thememory device 600 and other electrical components. Top contact 600 mayalso be used to form a mechanical connection between the memory device600 and another device.

In some embodiments, top barrier layer 620 includes at least one ofTitanium Nitride (TiN), Tantalum Nitride (TaN), and Titanium Tungsten(TiW). In alternative embodiments, one or more other materials are used.

Top barrier layer 620 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 630, anyretention layer 640, and the memory layer 650. Top barrier layer 620 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 600. Accordingly, topbarrier layer 620 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 630 into the top barrier layer620. In addition, top barrier layer 620 is configured to conductelectrical current between the top electrode layer 630 and the topcontact 610.

The top barrier layer 620 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 620experiences substantially no chemical reaction with the top electrode630, such that the characteristics of the top barrier layer 620 and thetop electrode 630 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 620 and the top electrode 630, such that thecharacteristics of the memory layer 650 and the retention layer 640remain substantially unaffected by one another.

In some embodiments, top electrode layer 630 includes at least one ofTungsten (W), Molybdenum (Mo), Nickel (Ni), Iron (Fe), Cobalt (Co), andChromium (Cr). In alternative embodiments, one or more other materialsare used. For example, another metal, conductive oxide, or otherconductive compound may be use.

Top electrode layer 630 forms an electrical connection between theretention layer 640 or the memory layer 650 and the top barrier layer620. Top electrode layer 630 is formed with a material which forms asecure bond with the retention layer 640 or the memory layer 650.

Top electrode layer 630 cooperatively forms a metal oxide heterojunctionmemory with memory layer 650, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 650 in response to anelectric field applied across the electrode layer 630 and the memorylayer 650. In some embodiments, the top electrode layer 630 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 650. In alternative embodiments, the topelectrode layer 630 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 650.

In some embodiments, optional retention layer 640 includes at least oneof SnOx, InOx, (In, Sn)Ox, and doped ZnO. In alternative embodiments,one or more other materials are used.

In some embodiments, retention layer 640 has high electricalconductivity electrical conductivity. For example, retention layer 640may have conductivity greater than 1 E-4 ohm-m. Retention layer 640 mayalso be resistant to conduction of oxygen ions and vacancies in responseto an applied electric field. In addition, voltage dependence of theionic conductivity of retention layer 640 may be highly non-linear.Furthermore, retention layer 640 may experience no chemical interactionwith the top electrode layer 630 and memory layer 650. Additionally,retention layer 640 may form an ohmic contact with top electrode 630.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 630and the memory layer 650. Retention layer 640 may be placed between thetop electrode layer 630 and the memory layer 650 and improves memorycell retention. Because retention layer 640 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 640 side of topelectrode layer 630 and the memory layer 650, and data retention isimproved. In addition, because retention layer 640 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 640.

The retention layer 640 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 640experiences substantially no chemical reaction with the memory layer650, such that the characteristics of the memory layer 650 and theretention layer 640 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 640 and the memory layer 650, such that thecharacteristics of the memory layer 650 and the retention layer 640remain substantially unaffected by one another.

In some embodiments, memory layer 650 includes at least one ofPraseodymium

Calcium Manganese Oxide or (Pr1-xCax)MnO₃ (PCMO), (Sm1-xCax)MnO₃, and(La1-xSrx)MnO₃. In alternative embodiments, one or more other materialsare used. In some embodiments, the memory layer 650 is between about 5nm and about 10 nm thick.

In some embodiments, conductive bottom barrier layer 670 includes atleast one of Titanium Nitride (TiN), Tantalum Nitride (TaN), andTitanium Tungsten (TiW). In alternative embodiments, one or more othermaterials are used. In some embodiments, conductive bottom barrier layer670 is formed of substantially the same material as the top barrierlayer 620.

Bottom barrier layer 670 may be formed of a material having a band gapwider than that of one or more of any retention layer 640, and thememory layer 650. Bottom barrier layer 670 is configured tosubstantially prevent the conduction of oxygen ions or vacancies duringoperation of the memory device 600. Accordingly, bottom barrier layer670 substantially prevents oxygen ions or vacancies from escaping fromthe memory layer 650 into the bottom barrier layer 670. In addition,bottom barrier layer 670 is configured to conduct electrical currentbetween the memory layer 650 and the bottom contact 680.

The bottom barrier layer 670 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, bottom barrier layer670 experiences substantially no chemical reaction with the bottomcontact 680, such that the characteristics of the bottom barrier layer670 and the bottom contact 680 remain substantially unaffected by oneanother. Also, in some embodiments, substantially no diffusion occursbetween the bottom barrier layer 670 and the bottom contact 680, suchthat the characteristics of the bottom barrier layer 670 and the bottomcontact 680 remain substantially unaffected by one another.

In some embodiments, bottom contact 680 includes at least one of Copper(Cu), Aluminum (Al), Tungsten (W), Ruthenium (Ru), Platinum (Pt),Iridium (Ir), and Rhodium (Rh). In alternative embodiments, one or moreother materials are used. In some embodiments, bottom contact 680 isformed of substantially the same material as the top contact 610.

In some embodiments, side barrier 690 includes at least one of AlOx,SiO₂, and Si₃N₄. In alternative embodiments, one or more other materialsare used.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 600, top barrier layer 620, bottom barrier layer 670,and side barrier layers 690 have little or substantially zero oxygen iondiffusion coefficients, such that the oxygen ions and vacancies areconfined to top electrode layer 630, retention layer 640 (if present),and memory layer 650, by top barrier layer 620, bottom barrier layer670, and side barrier layers 690. As a result, the reliability of memorydevice 600 is excellent.

The side barrier layers 690 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, side barrier layers690 experience substantially no chemical reaction with the other layers,such that the characteristics of the side barrier layers 690 and theother layers remain substantially unaffected by one another. Also, insome embodiments, substantially no diffusion occurs between the sidebarrier layers 690 and the other layers, such that the characteristicsof the side barrier layers 690 and the other layers remain substantiallyunaffected by one another.

In certain embodiments, bottom contact 680 is formed with Cu, conductivebottom barrier layer 670 is formed with TaN, memory layer 650 is formedwith PCMO, retention layer 640 is formed with SnO, top electrode layer630 is formed with W, top barrier layer 620 is formed with TaN, and topcontact 610 is formed with Cu.

In certain embodiments, bottom contact 680 is formed with Ru, conductivebottom barrier layer 670 is formed with TaN, memory layer 650 is formedwith PCMO, retention layer 640 is formed with doped ZnO, top electrodelayer 630 is formed with W, top barrier layer 620 is formed with TaN,and top contact 610 is formed with Ru.

In certain embodiments, bottom contact 680 is formed with W, conductivebottom barrier layer 670 is formed with TaN, memory layer 650 is formedwith (SmCa)MnO₃, retention layer 640 is formed with InOx, top electrodelayer 630 is formed with W, top barrier layer 620 is formed with TaN,and top contact 610 is formed with Cu.

FIG. 7 is a schematic illustration of a memory device 700 according toan embodiment. Memory device 700 includes bottom contact 780, conductivebottom barrier layer 770, memory layer 750, optional retention layer740, top electrode layer 730, top barrier layer 720, and top contact710.

Memory device 700 may be formed by forming bottom contact 780, formingconductive bottom barrier layer 770 on bottom contact 780, formingmemory layer 750 on conductive bottom barrier layer 770, optionallyforming retention layer 740 on memory layer 750, forming top electrodelayer 730 on retention layer 740 or on memory layer 750, forming topbarrier layer 720 on top electrode layer 730, and forming top contact710 on top barrier layer 720.

In some embodiments, each of the interfaces of the various layers ofmemory device 700 forms an ohmic contact between the layers.

Top contact 710 may have characteristics similar or identical to thoseof top contact 210 discussed elsewhere herein.

Top contact 710 is used to form an electrical connection between thememory device 700 and other electrical components. Top contact 700 mayalso be used to form a mechanical connection between the memory device700 and another device.

Top barrier layer 720 may have characteristics similar or identical tothose of top barrier layer 220 discussed elsewhere herein.

Top barrier layer 720 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 730, anyretention layer 740, and the memory layer 750. Top barrier layer 720 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 700. Accordingly, topbarrier layer 720 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 730 into the top barrier layer720. In addition, top barrier layer 720 is configured to conductelectrical current between the top electrode layer 730 and the topcontact 710.

The top barrier layer 720 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 720experiences substantially no chemical reaction with the top electrode730, such that the characteristics of the top barrier layer 720 and thetop electrode 730 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 720 and the top electrode 730, such that thecharacteristics of the memory layer 750 and the retention layer 740remain substantially unaffected by one another.

Top electrode layer 730 may have characteristics similar or identical tothose of top electrode layer 630 discussed elsewhere herein.

Top electrode layer 730 forms an electrical connection between theretention layer 740 or the memory layer 750 and the top barrier layer720. Top electrode layer 730 is formed with a material which forms asecure bond with the retention layer 740 or the memory layer 750.

Top electrode layer 730 cooperatively forms a metal oxide heterojunctionmemory with memory layer 750, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 750 in response to anelectric field applied across the electrode layer 730 and the memorylayer 750. In some embodiments, the top electrode layer 730 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 750. In alternative embodiments, the topelectrode layer 730 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 750.

Optional retention layer 740 may have characteristics similar oridentical to those of optional retention layer 240 discussed elsewhereherein.

In some embodiments, retention layer 740 may experience no chemicalinteraction with the top electrode layer 730 and memory layer 750.Additionally, retention layer 740 may form an ohmic contact with topelectrode 730.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 730and the memory layer 750. Retention layer 740 may be placed between thetop electrode layer 730 and the memory layer 750 and improves memorycell retention. Because retention layer 740 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 740 side of topelectrode layer 730 and the memory layer 750, and data retention isimproved. In addition, because retention layer 740 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 740.

The retention layer 740 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 740experiences substantially no chemical reaction with the memory layer750, such that the characteristics of the memory layer 750 and theretention layer 740 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 740 and the memory layer 750, such that thecharacteristics of the memory layer 750 and the retention layer 740remain substantially unaffected by one another.

Memory layer 750 may have characteristics similar or identical to thoseof memory layer 250 discussed elsewhere herein.

Conductive bottom barrier layer 770 may have characteristics similar oridentical to those of conductive bottom barrier layer 270 discussedelsewhere herein. In some embodiments, conductive bottom barrier layer770 is formed of substantially the same material as the top barrierlayer 720.

Bottom barrier layer 770 may be formed of a material having a band gapwider than that of one or more of any retention layer 740 and the memorylayer 750. Bottom barrier layer 770 is configured to substantiallyprevent the conduction of oxygen ions or vacancies during operation ofthe memory device 700. Accordingly, bottom barrier layer 770substantially prevents oxygen ions or vacancies from escaping from thememory layer 750 into the bottom barrier layer 770. In addition, bottombarrier layer 770 is configured to conduct electrical current betweenthe memory layer 750 and the bottom contact 780.

The bottom barrier layer 770 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, bottom barrier layer770 experiences substantially no chemical reaction with the bottomcontact 780, such that the characteristics of the bottom barrier layer770 and the bottom contact 780 remain substantially unaffected by oneanother. Also, in some embodiments, substantially no diffusion occursbetween the bottom barrier layer 770 and the bottom contact 780, suchthat the characteristics of the bottom barrier layer 770 and the bottomcontact 780 remain substantially unaffected by one another.

Bottom contact 780 may have characteristics similar or identical tothose of conductive bottom contact 280 discussed elsewhere herein. Insome embodiments, bottom contact 780 is formed of substantially the samematerial as the top contact 710.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 700, top barrier layer 720 and bottom barrier layer 770have little or substantially zero oxygen ion diffusion coefficients,such that the oxygen ions and vacancies are confined to top electrodelayer 730, retention layer 740 (if present), and memory layer 750 by topbarrier layer 720 and bottom barrier layer 770. As a result, thereliability of memory device 700 is excellent.

In certain embodiments, bottom contact 780 is formed with Cu, conductivebottom barrier layer 770 is formed with TaN, memory layer 750 is formedwith PCMO, retention layer 740 is formed with SnO, top electrode layer730 is formed with W, top barrier layer 720 is formed with TaN, and topcontact 710 is formed with Cu.

In certain embodiments, bottom contact 780 is formed with Ru, conductivebottom barrier layer 770 is formed with TaN, memory layer 750 is formedwith PCMO, retention layer 740 is formed with doped ZnO, top electrodelayer 730 is formed with W, top barrier layer 720 is formed with TaN,and top contact 710 is formed with Ru.

In certain embodiments, bottom contact 780 is formed with W, conductivebottom barrier layer 770 is formed with TaN, memory layer 750 is formedwith (SmCa)MnO3, retention layer 740 is formed with InOx, top electrodelayer 730 is formed with W, top barrier layer 720 is formed with TaN,and top contact 710 is formed with Cu.

FIG. 8 is a schematic illustration of a memory device 800 according toan embodiment. Memory device 800 includes memory layer 850, optionalretention layer 840, top electrode layer 830, top barrier layer 820, andtop contact 810.

Memory device 800 may be formed by forming memory layer 850, optionallyforming retention layer 840 on memory layer 850, forming top electrodelayer 830 on retention layer 840 or on memory layer 850, forming topbarrier layer 820 on top electrode layer 830, and forming top contact810 on top barrier layer 820.

In some embodiments, each of the interfaces of the various layers ofmemory device 800 forms an ohmic contact between the layers.

Top contact 810 may have characteristics similar or identical to thoseof top contact 210 discussed elsewhere herein.

Top contact 810 is used to form an electrical connection between thememory device 800 and other electrical components. Top contact 810 mayalso be used to form a mechanical connection between the memory device800 and another device.

Top barrier layer 820 may have characteristics similar or identical tothose of top barrier layer 220 discussed elsewhere herein.

Top barrier layer 820 may be formed of a material having a band gapwider than that of one or more of the top electrode layer 830, anyretention layer 840, and the memory layer 850. Top barrier layer 820 isconfigured to substantially prevent the conduction of oxygen ions orvacancies during operation of the memory device 800. Accordingly, topbarrier layer 820 substantially prevents oxygen ions or vacancies fromescaping from the top electrode layer 830 into the top barrier layer820. In addition, top barrier layer 820 is configured to conductelectrical current between the top electrode layer 830 and the topcontact 810.

The top barrier layer 820 may be formed using any deposition process,such as PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, top barrier layer 820experiences substantially no chemical reaction with the top electrode830, such that the characteristics of the top barrier layer 820 and thetop electrode 830 remain substantially unaffected by one another. Also,in some embodiments, substantially no diffusion occurs between the topbarrier layer 820 and the top electrode 830, such that thecharacteristics of the memory layer 850 and the retention layer 840remain substantially unaffected by one another.

Top electrode layer 830 may have characteristics similar or identical tothose of top electrode layer 230 discussed elsewhere herein.

Top electrode layer 830 forms an electrical connection between theretention layer 840 or the memory layer 850 and the top barrier layer820. Top electrode layer 830 is formed with a material which forms asecure bond with the retention layer 840 or the memory layer 850.

Top electrode layer 830 cooperatively forms a metal oxide heterojunctionmemory with memory layer 850, and is configured to accept or donateoxygen ions or vacancies from or to memory layer 850 in response to anelectric field applied across the electrode layer 830 and the memorylayer 850. In some embodiments, the top electrode layer 830 may beoxygen-rich and may cooperatively form an oxygen ion heterojunctionmemory cell with memory layer 850. In alternative embodiments, the topelectrode layer 830 may be oxygen depleted and may cooperatively form anoxygen vacancy heterojunction memory cell with memory layer 850.

Optional retention layer 840 may have characteristics similar oridentical to those of optional retention layer 240 discussed elsewhereherein.

In some embodiments, retention layer 840 may experience no chemicalinteraction with the top electrode layer 830 and memory layer 850.Additionally, retention layer 840 may form an ohmic contact with topelectrode 830.

Data retention in the memory cell is greatly influenced by the diffusionof oxygen ions and oxygen vacancies between the top electrode layer 830and the memory layer 850. Retention layer 840 may be placed between thetop electrode layer 830 and the memory layer 850 and improves memorycell retention. Because retention layer 840 is resistant to conductionof oxygen ions and vacancies, oxygen ions and vacancies are less likelyto diffuse between the oxide on the retention layer 840 side of topelectrode layer 830 and the memory layer 850, and data retention isimproved. In addition, because retention layer 840 is electricallyconductive, electrical performance of the memory cell experiences littleor substantially no degradation as a consequence of retention layer 840.

The retention layer 840 may be formed using any deposition process, suchas PVD, CVD, sputtering, evaporation, ALD, or another deposition orgrowth process. Furthermore, in some embodiments, retention layer 840experiences substantially no chemical reaction with the memory layer850, such that the characteristics of the memory layer 850 and theretention layer 840 remain substantially unaffected by one another.Also, in some embodiments, substantially no diffusion occurs between theretention layer 840 and the memory layer 850, such that thecharacteristics of the memory layer 850 and the retention layer 840remain substantially unaffected by one another.

Memory layer 850 may have characteristics similar or identical to thoseof memory layer 250 discussed elsewhere herein.

Reliability of interface switching memories which conduct ions andvacancies between layers depends critically on losses of the criticalspecies from the cell. Therefore, techniques to prevent any losses ofthe critical species from the cell during the cycling and retention arebeneficial.

In memory device 800, top barrier layer 820 has little or asubstantially zero oxygen ion diffusion coefficient, such that theoxygen ions and vacancies are confined to top electrode layer 830,retention layer 840 (if present), and memory layer 850 by top barrierlayer 820. As a result, the reliability of memory device 800 isexcellent.

The cost of memories using an array of memory devices as describedherein is much less than that of memories which use traditionalnon-volatile memory cells, such as DRAM cells. This is the case at leastbecause of the following differences resulting from one or more of thefeatures discussed herein as understood by those of skill in the art: 1)Memory devices discussed herein have area that is much smaller than DRAMcells, 2) The manufacturing process for making DRAM cells typicallyincludes forming a trench in the substrate, for example, for forming acapacitor, while memory devices such as memory device 100 may bemanufactured without forming a trench.

The speed or access time of memories using an array of memory devices asdescribed herein is much better than that of memories which usetraditional non-volatile memory cells. This is the case at least becausethe electrical resistance of the layers and contacts outside of thememory layer is low, as discussed above with reference to each of thelayers and contacts. Memory speed using memory devices as describedherein is also improved over traditional memories because large memorysystems using memory devices as described herein may be operated withoutspeed crippling Error Correction Code (ECC) techniques as a result, forexample, of reliable retention of the memory states of the memorydevices. For example, memory systems having Megabyte, Gigabyte, Terabytestorage may be operated without speed crippling ECC techniques.

Though the present invention is disclosed by way of specific embodimentsas described above, those embodiments are not intended to limit thepresent invention. Based on the methods and the technical aspectsdisclosed above, variations and changes may be made to the presentedembodiments by those skilled in the art without departing from thespirit and the scope of the present invention.

What is claimed is:
 1. A memory device, comprising: a bottom contact; amemory layer connected to the bottom contact, wherein the memory layerhas a variable resistance; a conductive top electrode on the memorylayer, wherein the top electrode and the memory layer cooperatively forma heterojunction memory structure; and a retention layer between thememory layer and the top electrode, wherein the retention layer has anionic conductivity which varies non-linearly with voltage.
 2. The memorydevice of claim 1, wherein the retention layer substantially does notchemically react with at least one of the memory layer and the topelectrode.
 3. The memory device of claim 1, wherein diffusion betweenthe retention layer and at least one of the memory layer and the topelectrode does not occur.
 4. The memory device of claim 1, wherein afirst contact is formed at an interface between the retention layer andthe memory layer, wherein a second contact is formed at an interfacebetween the retention layer and the top electrode, and wherein at leastone of the first and second contacts is ohmic.
 5. The memory device ofclaim 1, further comprising: a first barrier layer, configured tosubstantially prevent conduction of ions or vacancies therethrough,wherein the top electrode is between the first barrier layer and theretention layer; and a second barrier layer, configured to substantiallyprevent conduction of ions or vacancies therethrough, wherein the memorylayer is between the second barrier layer and the retention layer. 6.The memory device of claim 5, further comprising a side barrier layer,wherein the first and second barrier layers and the side barrier layerdefine an enclosed space, wherein the top electrode, the retentionlayer, and the memory layer are within the space, and wherein ions ofthe top electrode, the retention layer, and the memory layer areconfined to the space by the first and second barrier layers and theside barrier layer.
 7. The memory device of claim 1, wherein a memorystate of the memory device may be determined by applying a read voltageacross the bottom contact and the top electrode, wherein the readvoltage is less than about 0.5 volts.
 8. The memory device of claim 1,wherein the memory layer comprises a semiconductor material.
 9. A methodof manufacturing a memory device, comprising: forming a bottom contact;connecting a memory layer to the bottom contact, wherein the memorylayer has a variable resistance; forming a conductive top electrode onthe memory layer, wherein the top electrode and the memory layercooperatively form a heterojunction memory structure; and forming aretention layer between the memory layer and the top electrode, whereinthe retention layer has an ionic conductivity which varies non-linearlywith voltage.
 10. The method of claim 9, wherein the retention layersubstantially does not chemically react with at least one of the memorylayer and the top electrode.
 11. The method of claim 9, whereindiffusion between the retention layer and at least one of the memorylayer and the top electrode does not occur.
 12. The method of claim 9,wherein a first contact is formed at an interface between the retentionlayer and the memory layer, wherein a second contact is formed at aninterface between the retention layer and the top electrode, and whereinat least one of the first and second contacts is ohmic.
 13. The methodof claim 9, further comprising: forming a first barrier layer,configured to substantially prevent conduction of ions or vacanciestherethrough, wherein the top electrode is between the first barrierlayer and the retention layer; and forming a second barrier layer,configured to substantially prevent conduction of ions or vacanciestherethrough, wherein the memory layer is between the second barrierlayer and the retention layer.
 14. The method of claim 13, furthercomprising forming a side barrier layer, wherein the first and secondbarrier layers and the side barrier layer define an enclosed space,wherein the top electrode, the retention layer, and the memory layer arewithin the space, and wherein ions of the top electrode, the retentionlayer, and the memory layer are confined to the space by the first andsecond barrier layers and the side barrier layer.
 15. The method ofclaim 9, wherein a memory state of the memory device may be determinedby applying a read voltage across the bottom contact and the topelectrode, wherein the read voltage is less than about 0.5 volts. 16.The method of claim 9, wherein the memory layer comprises asemiconductor material.
 17. A method of using a memory device, thememory device comprising a bottom contact, a memory layer with avariable resistance connected to the bottom contact, a conductive topelectrode on the memory layer, and a retention layer between the memorylayer and the top electrode, wherein the top electrode and the memorylayer cooperatively form a heterojunction memory structure, and whereinthe retention layer has an ionic conductivity which varies non-linearlywith voltage, the method comprising: applying a first voltage differenceacross the bottom contact and the top electrode, whereby an electricfield is generated in the memory layer, and such that a resistivitystate of the memory layer is changed; applying a second voltagedifference across the bottom contact and the top electrode; while thesecond voltage difference is applied, causing a first current to beconducted through the bottom contact, the memory layer, the retentionlayer, and the top electrode; and determining the resistivity state ofthe memory layer based on the second voltage difference and the firstcurrent.
 18. The method of claim 17, wherein the memory device furthercomprises: a first barrier layer, configured to substantially preventconduction of ions or vacancies therethrough, wherein the top electrodeis between the first barrier layer and the retention layer; and a secondbarrier layer, configured to substantially prevent conduction of ions orvacancies therethrough, wherein the memory layer is between the secondbarrier layer and the retention layer, wherein the method furthercomprises causing the first current to be conducted through the firstand second barrier layers.
 19. The method of claim 18, wherein thememory device further comprises a side barrier layer, wherein the firstand second barrier layers and the side barrier layer define an enclosedspace, wherein the top electrode, the retention layer, and the memorylayer are within the space, and wherein the method further comprisesconfining ions of the top electrode and the memory layer to the spacewith the first and second barrier layers and the side barrier layer. 20.The method of claim 17, wherein the second voltage difference is lessthan about 0.5 volts.
 21. The method of claim 17, wherein the memorylayer comprises a semiconductor material.